Objection 1. It would seem that not only a bishop confers the sacrament of Order. For the imposition of hands has something to do with the consecration. Now not only the bishop but also the assisting priests lay hands on the priests who are being ordained. Therefore not only a bishop confers the sacrament of Order.
Objection 2. Further, a man receives the power of Order, when that which pertains to the act of his Order is handed to him. Now the cruet with water, bowl* and towel, are given to the subdeacon by the archdeacon; as also the candlestick with candle, and the empty cruet to the acolyte. ["Bacili." The rubric has "aquamanili." Some texts of the Summa have "mantili" ("maniple"), but the archdeacon does not give the maniple to the subdeacon.] Therefore not only the bishop confers the sacrament of Order.
Objection 3. Further, that which belongs to an Order cannot be entrusted to one who has not the Order. Now the conferring of minor Orders is entrusted to certain persons who are not bishops, for instance to Cardinal priests. Therefore the conferring of Orders does not belong to the episcopal Order.
Objection 4. Further, whoever is entrusted with the principal is entrusted with the accessory also. Now the sacrament of Order is directed to the Eucharist, as accessory to principal. Since then a priest consecrates the Eucharist, he can also confer Orders.
Objection 5. Further, there is a greater distinction between a priest and a deacon than between bishop and bishop. But a bishop can consecrate a bishop. Therefore a priest can ordain a deacon.
On the contrary, Ministers are applied by their Orders to the Divine worship in a more noble way than the sacred vessels. But the consecration of the vessels belongs to a bishop only. Much more therefore does the consecration of ministers.
Further, the sacrament of Order ranks higher than the sacrament of Confirmation. Now a bishop alone confirms. Much more therefore does a bishop alone confer the sacrament of Order.
Further, virgins are not placed in a degree of spiritual power by their consecration, as the ordained are. Yet a bishop alone can consecrate a virgin. Therefore much more can he alone ordain.
I answer that, The episcopal power stands in the same relation to the power of the lower Orders, as political science, which seeks the common good, to the lower acts and virtues which seek some special good, as appears from what was said above (Question 37, Article 1). Now political science, as stated in Ethic. i, 2, lays down the law to lower sciences, namely what science each one ought to cultivate, and how far he should pursue it and in what way. Wherefore it belongs to a bishop to assign others to places in all the Divine services. Hence he alone confirms, because those who are confirmed receive the office, as it were, of confessing the faith; again he alone blesses virgins who are images of the Church, Christ's spouse, the care of which is entrusted chiefly to him; and he it is who consecrates the candidates for ordination to the ministry of Orders, and, by his consecration, appoints the vessels that they are to use; even as secular offices in various cities are allotted by him who holds the highest power, for instance by the king.
Reply to Objection 1. As stated above (Question 37, Article 5), at the imposition of hands there is given, not the character of the priestly Order, but grace which makes a man fit to exercise his Order. And since those who are raised to the priesthood need most copious grace, the priests together with the bishop lay hands on them, but the bishop alone lays hands on deacons.
Reply to Objection 2. Since the archdeacon is as it were minister-in-chief, all things pertaining to the ministry are handed by him, for instance the candle with which the acolyte serves the deacon by carrying it before him at the Gospel, and the cruet with which he serves the subdeacon; and in like manner he gives the subdeacon the things with which the latter serves the higher Orders. And yet the principal act of the subdeacon does not consist in these things, but in his co-operation as regards the matter of the sacrament; wherefore he receives the character through the chalice being handed to him by the bishop. On the other hand, the acolyte receives the character by virtue of the words of the bishop when the aforesaid things--the cruet rather than the candlestick--are handed to him by the archdeacon. Hence it does not follow that the archdeacon ordains.
Reply to Objection 3. The Pope, who has the fulness of episcopal power, can entrust one who is not a bishop with things pertaining to the episcopal dignity, provided they bear no immediate relation to the true body of Christ. Hence by virtue of his commission a simple priest can confer the minor Orders and confirm; but not one who is not a priest. Nor can a priest confer the higher Orders which bear an immediate relation to Christ's body, over the consecration of which the Pope's power is no greater than that of a simple priest.
Reply to Objection 4. Although the Eucharist is in itself the greatest of the sacraments, it does not place a man in an office as does the sacrament of Order. Hence the comparison fails.
Reply to Objection 5. In order to bestow what one has on another, it is necessary not only to be near him but also to have fulness of power. And since a priest has not fulness of power in the hierarchical offices, as a bishop has, it does not follow that he can raise others to the diaconate, although the latter Order is near to his.
Source: Summa Theologica (New Advent)